It is known that, in the techniques of coating sheets of paper, cardboard or the like, there is deposited on the support surface or surfaces a coating composition which can contain one or more mineral fillers, one or more binders and various additives.
Amongst these additives are water-retention agents, not to be confused with agents for the retention of fines or filler on the wire during the manufacture of the paper.
A composition for coating paper generally consists of a filler, which can be one or more pigments, one or more polymer binders and various additives such as in particular a lubricant such as a calcium stearate, a wax or a fatty acid ester, and possibly antifoaming agents and the like, well-known to persons skilled in the art.
After deposition on the support, the coating color has a natural tendency to transfer into the support all or part of the water and the water-soluble parts which it contains. It is necessary to control excessively rapid migration, which would impair the physical and optical characteristics of the coating.
It is therefore sought to retain the water in the coating composition or coating color. Thus this water retention must be controlled, in order to prevent changes in the rheology of the unused coating color recycled in the coating process.
Use is habitually made, as a water-retention agent, of natural or synthetic agents such as carboxymethylcellulose (CMC), starch, polyvinyl alcohols (PVAs) or certain latexes or emulsions of polymers with a high carboxyl content or polycarboxylates, for example of the polyacrylate type. Such substances are described, for example, in the patent EP 0 509 878.
A particular class of retention agent is known as alkali swellable polymers, and is described in the aforementioned patent, and as prior art in U.S. Pat. No. 4,780,500, which describes water-retention agents consisting mainly of acrylic acid monomer and secondarily itaconic acid monomer, with a molecular weight of 100,000 to 800,000.
It is also known that known products of the PVA or CMC type have limited use because of the high viscosities which they develop in the coating colors in parallel to their efficacy with respect to water-retention.
Thus such additives pose the following problems:                the need for compatibility with the other ingredients in the coating color, such as the pigments such as carbonates, kaolin and the like, or binders such as latexes of the SBR type (styrene-butadiene-rubber latex), and other ingredients well know to persons skilled in the art;        increase in the viscosity up to values which are not optimal for the preparation and use of the coating color;        risk of affecting machinability;        risk of affecting the properties of the coating color or properties of the manufactured coated paper, such as a good gloss;        coating splashes;        the formation of dribbles during the deposition which may go as far as the presence of scratches on the coated paper, also referred to as the phenomenon of “bleading”.        
The problem posed by the current inadequacies of the technology in this regard is therefore managing to reconcile often contradictory requirements, such as in particular the need for excellent water-retention in the coating color, which, according to current knowledge, can be obtained only by the addition of additives increasing the viscosity of the coating color but which may pose problems of machinability and also problems because, by hypothesis, the viscosity of the coating color becomes too high compared with the optimum value which would be desirable.
The invention relates in particular to agents aimed at obtaining the above properties.
It has been sought in the prior art to resolve this problem in various ways, particularly in the following documents, which can be classified in several categories.
It has notably been sought in the prior art to produce agents consisting of non water-soluble copolymers, such as for example in the German patent document DD 231 819, which concerns to binders adapted to coating colors and which procure effects which are unfavourable to the rheological properties.
U.S. Pat. No. 4,474,919 also describes non water-soluble agents of the latex type containing styrene and for example acrylic acid.
Likewise, the patent FR 2 675 165 relates to a composition for the coating of paper with an alkaline pH comprising a mixture of two insoluble latexes with special granulometries, these granulometries forming the essential teachings of this patent, and a mineral pigment, also clearly specified, aimed at improving water retention and machinability, with a “suitable” viscosity. It should be noted that the granulometry is also the main teaching of the patent FR 2 740 456.
Likewise, the document JP 58-054096 is known, which describes agents which, according to the proportions of their components, are latexes.
The patent JP-56101996 can also be cited, which, relating to a mixture of a styrene and maleic acid semi-ester copolymer and a terpolymer of styrene, acrylic acid and acrylic ester, says nothing about the simultaneous control of the viscosity and water retention of the coating color.
The same analysis applies to the patent JP-56101995.
Likewise, the patent DD 151 463 describes a viscosity regulating latex which is of the conventional binder type and relates only to the improvement in the viscosity and not the water retention.
U.S. Pat. No. 5,650,458 also describes compositions based on mixtures of styrene polymer and possibly acrylic acid, butyl-benzyl phthalate, butadiene-styrene latex, and a styrene and acrylic acid copolymer. This document relates only to the water retention and the quality of the final paper.
Equally, the patent JP-55018423 describes an acrylic acid copolymer and other monomers such as styrene in order to produce a simple dispersant which improves only the flow characteristics of the coating color.
Thus styrene and (meth)acrylic monomers are naturally monomers known in general terms in the field of coating colors for paper, and also in many other fields such as dispersants, agents for paint, etc.
As can be seen from a reading of the above analysis, the prior art does not particularly direct persons skilled in the art towards a choice of comonomers for resolving the problems the solution of which the invention relates to.
On the contrary, the prior art shows that many comonomers are used for different or very specific purposes, such as the viscosity properties, or water retention, or as dispersants, or as conventional binders, or co-binders etc, and for preparing both water-soluble and insoluble copolymers.